1. Field of the Invention
The present invention relates to a liquid crystal display device comprising a first substrate having a pixel electrode and a first alignment film, and a second substrate having a common electrode and a second alignment film.
2. Prior Art
Recently, as liquid crystal display devices in which a light transmissivity and a viewing angle can effectively be improved, liquid crystal display devices adopting a multiple domain vertical alignment (or SVA; Super Vertical Alignment) method have come into wide use. As an example of such conventional liquid crystal display devices in which the SVA method has been adopted, a liquid crystal display device having a TFT substrate with a TFT (thin film transistor) formed thereon and a color filter substrate with a color filter formed thereon will hereinafter be described.
FIGS. 6 through 9 are schematic explanatory views of a conventional liquid crystal display device adopting the SVA method.
FIG. 6 includes an enlarged plan view (A) of a portion of the TFT substrate in this conventional liquid crystal display device which corresponds to one pixel, and a cross-sectional view (B) of the portion taken along line Axe2x80x94A. FIG. 6(A) shows only a gate bus, source buses and a pixel electrode, and a TFT is omitted from the drawing.
As shown in FIG. 6(A), this TFT substrate 10 is formed with a gate bus 11 and source buses 12. Further, the TFT substrate 10 is formed with a pixel electrode 13 and a TFT (not shown) correspondingly to each pixel. Slits 13b (portions indicated by multiple dots) are formed at the central part of the pixel electrode 13. Further, slits 13a and slits 13c (portions indicated by multiple dots) are formed at the upper and lower portions (in the drawing) of the pixel electrode 13, respectively. Gaps 14 are provided between the source buses 12 and the pixel electrode 13.
As shown in FIG. 6(B), the surface of the TFT substrate 10 is covered with an alignment film 15. This alignment film 15 is omitted in FIG. 6(A). The TFT substrate 10 is opposed to a color filter substrate via a liquid crystal layer.
FIG. 7 includes a plan view (A) of the TFT substrate and the color filter substrate opposed to each other via the liquid crystal layer as viewed from the color filter substrate side, and a cross-sectional view (B) of these two substrates taken along line Bxe2x80x94B. To illustrate characteristic portions of this conventional liquid crystal display device in a simplified manner, FIG. 7(A) shows only the gate bus, the source buses, the pixel electrode and protrusions (or ridges).
As shown in FIG. 7(B), the TFT substrate 10 is opposed to the color filter substrate 20 via the liquid crystal layer 30. This liquid crystal layer 30 is constituted of negative liquid crystal molecules having properties to be aligned perpendicularly to electric force lines.
The color filter substrate 20 is provided with a color filter (not shown). Further, the color filter substrate 20 is provided with a common electrode 21 on which protrusions (or ridges) 22 are formed. As shown in FIG. 7(A), these projections 22 are formed on the right-hand and left-hand sides of the slits 13b in parallel thereto. A material for these protrusions 22 may be selected, for example, from phenolic resins, novolac resins, and acrylic resins. Further, as shown in FIG. 7(B), the common electrode 21 and the protrusions 22 are covered by an alignment film 23. In this way, protrusions 22 are formed between the common electrode 21 and the alignment film 23, so that the surface of the color filter substrate 20 is formed with portions 24 which are projected toward the liquid crystal layer 30 caused by the protrusions 22. Since the protrusions 22 are formed in parallel to the slits 13b as shown in FIG. 7(A), the projected portions 24 are formed also in parallel to the slits 13b. 
The alignment films 15 and 23 formed on the TFT substrate 10 and the color filter substrate 20, respectively, are adapted to align liquid crystal molecules perpendicularly to these alignment films 15 and 23, when no voltage is applied to the liquid crystal layer 30.
Description will now be made on the behavior of the liquid crystal molecules when a voltage is applied between the substrates 10 and 20 with reference to FIG. 8 and FIG. 9 showing the liquid crystal molecules more distinctly.
FIG. 8 is a cross-sectional view of the device taken along line Cxe2x80x94C when no voltage is applied between the substrates 10 and 20 in FIG. 7, and FIG. 9 is the same cross-sectional view when a voltage is applied between the substrates 10 and 20 in FIG. 7. The liquid crystal molecules are indicated by ellipses.
As shown in FIG. 8, when no voltage is applied (hereinafter referred to as xe2x80x9cvoltage non-applied periodxe2x80x9d), the liquid crystal molecules in the liquid crystal layer 30 are oriented perpendicularly to the alignment film 23 (i.e., to each of the substrates 10 and 20). In the state that the liquid crystal molecules are perpendicularly oriented, when a voltage is applied, electric force lines as represented by broken lines develop. As the liquid crystal molecules constituting the liquid crystal layer 30 are negative liquid crystal molecules, they start to be inclined perpendicularly to the electric force lines (horizontally with respect to the substrates 10 and 20). In this case, the electric force lines develop substantially perpendicularly to the substrates 10 and 20. However, as the slits 13b (See FIG. 6) are provided in the pixel electrode 13, and the gap 14 (See FIG. 6) is provided between the pixel electrode 13 and the source bus 12, the electric force lines around the slit 13b and the gap 14 are slightly bent and enter/leave the pixel electrode 13. Accordingly, immediately after the development of these electric force lines, the electric force lines enter/leave those liquid crystal molecules present in positions away from the slit 13b and the gap 14 substantially in parallel thereto, but enter/leave at a slightly inclined angle those liquid crystal molecules present in positions around the slit 13b and the gap 14 under the influence of the slit 13b and the gap 14. Therefore, the liquid crystal molecules 31 and 32 present around the slit 13b and the gap 14 start to be inclined horizontally to the substrates 10 and 20 earlier than the liquid crystal molecules present in the positions away from the slit 13b and the gap 14. When the liquid crystal molecules 31 and 32 start to be inclined, the other liquid crystal molecules sequentially start to be inclined from the liquid crystal molecules 31 and 32 as their starting points. In this case, when considering the directions of the electric force lines that enter/leave the respective liquid crystal molecules 31 and 32, the liquid crystal molecule 31 starts to be oriented perpendicularly to the electric force line while being inclined in the clockwise direction T, whereas the liquid crystal molecule 32 starts to be oriented perpendicularly to the electric force line while being inclined in the counterclockwise direction Txe2x80x2. Accordingly, the liquid crystal molecules positioned in a region A closer to the slit 13b than the gap 14 are greatly influenced by the liquid crystal molecule 31 inclined in the clockwise direction T, and sequentially become inclined in the clockwise direction T. On the other hand, the liquid crystal molecules existing in a region B closer to the gap 14 are greatly influenced by the liquid crystal molecule 32 inclined in the counterclockwise direction Txe2x80x2, and sequentially become inclined in the counterclockwise direction Txe2x80x2. As a result, during the voltage-applied period, the directions of inclination of the liquid crystal molecules in the regions A and B are opposite to each other, and the liquid crystal molecules are oriented as shown in FIG. 9.
As shown in FIG. 9, when the liquid crystal molecules are inclined in opposite directions in the regions A and B, the boundary between the regions A and B becomes a disclination line, which reduces the light transmittance.
It is therefore an object of the present invention to provide a liquid crystal display device which has an improved light transmissivity of a liquid crystal layer when a voltage is applied.
To achieve the above object, the liquid crystal display device of the present invention comprises a first substrate having a pixel electrode and a first alignment film, and a second substrate having a common electrode and a second alignment film, the first substrate and the second substrate sandwiching a liquid crystal layer therebetween, and is characterized in that
said pixel electrode has at least one first slit, or said first substrate has at least one first protrusion between the pixel electrode and the first alignment film,
and in that the common electrode has a second slit extending in a direction different from a direction in which the first slit or the first protrusion extends, or the second substrate has a second protrusion between the common electrode and the second alignment film, the second slit or the second protrusion extending in a direction different from the direction in which the first slit or the first protrusion extends.
In the liquid crystal display device of the present invention, the common electrode has the second slit extending in a direction different from the direction in which the first slit or the first protrusion extends, or the second substrate has the second protrusion between the common electrode and the second alignment film, the second protrusion extending in a direction different from the direction in which the first slit or the first protrusion extends. In this way, the second slit or the second protrusion of the second substrate extends in a direction different from the direction in which the first slit or the first protrusion of the first substrate extends, as a result of which it becomes possible to align the liquid crystal molecules in a desired direction when a voltage is applied to the liquid crystal layer (the manner in which the alignment of liquid crystal molecules is controlled will be described later in detail with reference to some embodiments of the invention). Thus, it becomes possible to control the alignment of the liquid crystal molecules such that the transmissivity of light through the liquid crystal layer is improved when a voltage is applied to the liquid crystal layer.
In the liquid crystal display device of the present invention, it is preferable that the first substrate has a gate bus and a source bus, and that the second slit or the second protrusion (or the first slit or the first protrusion) is parallel to at least one of the gate bus and source bus.
By forming the second slit or the second protrusion in parallel to at least one of the gate bus or the source bus, it becomes possible to control the alignment of the liquid crystal molecules such that the transmissivity of light through the liquid crystal layer is improved when a voltage is applied to the liquid crystal layer.
In the liquid crystal display device of the present invention, it is preferable that the second slit or the second protrusion is formed at a position which opposes a central portion of the pixel electrode.
By forming the second slit or the second protrusion at the position opposing the central portion of the pixel electrode, it also becomes possible to control the alignment of the liquid crystal molecules such that the transmissivity of light through the liquid crystal layer is improved.